Semiconductors are now produced having several million transistors per die. To fit all of these devices on a single die, feature sizes must be minute. For example, a width of interconnect lines and a spacing between such lines needs to be very small. Accordingly, advances in semiconductor manufacturing allowing for more dense devices have resulted in the need for accurate, high resolution photolithography.
In general, photolithography involves transferring a mask pattern from a mask reticle onto a photoresist-coated substrate. The transfer is done using an imaging lens system and a beam of light. The reticle is often made from a slice of transparent quartz. The mask pattern on the reticle is a design that can be made up of opaque chromium regions and transparent quartz regions. If the beam of light is composed of coherent light rays, the mask pattern would be transferred exactly to the coated substrate.
However, light rays are not coherent and diffraction causes light to bend as it passes through the pattern reticle. Diffraction becomes significantly problematic when the chromium and transparent regions on the mask reticle are near in size to the wavelength of a beam of light. When diffraction occurs, regions on the coated substrate which should be dark receive the diffracted light. Thus, the pattern is adversely affected by diffracted light.
Phase shifting lithography is a method of reducing the effects of light diffraction. Adjacent transparent quartz regions of a pattern on an alternating phase shifting mask, which are separated by an opaque region, are created such that the phase of the light going through one of the transparent regions is shifted, or delayed, 180 degrees from that of light going through an adjacent transparent region. Shifting light by 180 degrees mitigates the problem caused by the diffraction.
When light is diffracted from two adjacent transparent regions onto a dark region of the substrate, the diffracted light cancels each other out. The cancellation occurs due to the additive nature of light rays: two rays having a phase difference of 180 degrees result in no light. Thus, shifting the phase of the light helps to ensure that the dark region remains dark.
One way to accomplish phase shifting is to etch every other transparent region on the quartz mask reticle so that light traveling through the etched regions will exit the reticle one-half wavelength behind light traveling through the unetched regions. However, it is difficult to etch the quartz reticle so that light has a phase shift of exactly 180 degrees. When a phase of light is not fully shifted 180 degrees, the alternating phase shifting mask has a phase error and diffraction remains a problem. Generally, when an alternating phase shifting mask has a phase shift error, the mask is discarded.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art to mitigate the problems of a phase shift error present in an alternating phase shifting mask.